Advances in T cell biology have led to numerous experimental strategies to prevent rejection or to induce transplantation tolerance. Unfortunately, these approaches have met with less success in the cinical setting. While differences in the immune systems of inbred laboratory rodents and heterogeneous human populations account for many of these discrepancies, relatively little attention has been given to important differences between experimental transplant models and the practice of clinical transplantation. In most animal experiments, organs are removed from unmanipulated healthy donors under ideal surgical conditions and transplanted immediately into healthy recipients. This simulates most closely the clinical conditions of a renal transplant performed from a living donor. However, hearts are harvested from cadaver donors. Unlike the carefully screened and controlled living donor, cadaveric organs are harvested under conditions of extreme physiologic stress many of which can stimulate components of the innate immune system. Although many of the alloantigen independent processes that contribute to allograft injury are poorly understood, we and others have demonstrated that complement (C) can augment allograft injury. This grant is focused on clinically relevant mechanisms by which C6 has a significant impact on allograft injury. This continuation application is based on our novel finding that a deficiency of C6, which prevents assembly of the membrane attack complex (MAC), can delay acute allograft rejection from 7-10 days to greater than 6 weeks. The specific aims are to test mechanisms that control C6 production and activation in allografts by: 1) Detrmining the contribution of C-Reactive Protein to C6 activation; 2) Inhibiting C6 production and function perioperatively; 3) Dissecting mechanisms controlling macrophage production of C6; 4) Dissecting mechanisms controlling regulation of MAC deposition; and 5) Verifying relevance of C6 in clinical samples by demonstrating. The experimental approach will utilize the novel C6 deficient strains of rats that we developed in the first funding period of this project for both in vivo cardiac transplant studies and as sources of C6 deficient cells for in vitro studies. Most importantly, we will use our extensive clinical material to verify the relevance of our experimental findings to humans.